ON MARCH 27, 1981, CBS radio news quoted a NASA scientist as saying
that engineers will be able to build self-replicating robots
within twenty years, for use in space or on Earth. These machines
would build copies of themselves, and the copies would be
directed to make useful products. He had no doubt of their
possibility, only of when they will be built. He was quite right.

Since 1951, when John von Neumann outlined the principles of
self-replicating machines, scientists have generally acknowledged
their possibility. In 1953 Watson
and Crick
described the structure of DNA,
which showed how living things pass on the instructions that
guide their construction. Biologists have since learned in
increasing detail how the self-replicating molecular machinery of
the cell works. They find
that it follows the principles von Neumann had outlined. As birds
prove the possibility of flight, so life in general proves the
possibility of self-replication, at least by systems of molecular
machines. The NASA scientist, however, had something else in
mind.

The ancient myth of a magical life-force (coupled with the
misconception that the increase of entropy means that
everything in the universe must constantly run down) has spawned
a meme saying that
replicators must violate some natural law. This simply isn't so.
Biochemists understand how cells replicate and they find no magic
in them. Instead, they find machines supplied with all the
materials, energy, and instructions needed to do the job. Cells do
replicate; robots could replicate.

Advances in automation will lead naturally toward mechanical
replicators, whether or not anyone makes them a specific goal. As
competitive pressures force increased automation, the need for
human labor in factories will shrink. Fujitsu Fanuc already runs
the machining section in a manufacturing plant twenty-four hours
a day with only nineteen workers on the floor during the day
shift, and none on the floor during the night shift.
This factory produces 250 machines a month, of which 100 are
robots.

Eventually, robots could do all the robot-assembly work, assemble
other equipment, make the needed parts, run the mines and
generators that supply the various factories with materials and
power, and so forth. Though such a network of factories spread
across the landscape wouldn't resemble a pregnant robot, it would
form a self-expanding, self-replicating system. The assembler breakthrough
will surely arrive before the complete automation of industry,
yet modern moves in this direction are moves toward a sort of
gigantic, clanking replicator.

But how can such a system be maintained and repaired without
human labor?

Imagine an automatic factory able to both test parts and assemble
equipment. Bad parts fail the tests and are thrown out or
recycled. If the factory can also take machines apart, repairs
are easy: simply disassemble the faulty machines, test all their
parts, replace any worn or broken parts, and reassemble them. A
more efficient system would diagnose problems without testing
every part, but this isn't strictly necessary.

A sprawling system of factories staffed by robots would be
workable but cumbersome. Using clever design and a minimum of
different parts and materials, engineers could fit a replicating
system into a single box - but the box might still be huge,
because it must contain equipment able to make and assemble many
different parts. How many different parts? As many as it itself
contains. How many different parts and materials would be needed
to build a machine able to make and assemble so many different
materials and parts? This is hard to estimate, but systems based
on today's technology would use electronic
chips. Making these alone would require too much equipment to
stuff into the belly of a small replicator.

Rabbits replicate, but they require prefabricated parts such as
vitamin molecules. Getting these from food lets them survive with
less molecular machinery than they would need to make everything
from scratch. Similarly, a mechanical replicator using
prefabricated chips could be made somewhat simpler than one that
made everything it needed. Its peculiar "dietary"
requirements would also tie it to a wider "ecology" of
machines, helping to keep it on a firm leash. Engineers in
NASA-sponsored studies have proposed using such semireplicators
in space, allowing space industry to expand with only a small
input of sophisticated parts from Earth.

Still, since bulk-technology replicators must make and assemble
their parts, they must contain both part-making and
part-assembling machines. This highlights an advantage of
molecular replicators: their parts are atoms, and atoms come
ready-made.

Molecular
Replicators

Cells replicate. Their machines copy their DNA, which directs
their ribosomal machinery to build other machines from simpler
molecules. These machines and molecules are held in a
fluid-filled bag. Its membrane lets in fuel molecules and parts
for more nanomachines, DNA, membrane, and so forth; it lets out
spent fuel and scrapped components. A cell replicates by copying
the parts inside its membrane bag, sorting them into two clumps,
and then pinching the bag in two. Artificial replicators could be
built to work in a similar way, but using assemblers instead of
ribosomes. In this way, we could build cell-like replicators that
are not limited to molecular machinery made from the soft, moist
folds of protein molecules.

But engineers seem more likely to develop other approaches to
replication. Evolution
had no easy way to alter the fundamental pattern of the cell, and
this pattern has shortcomings. In synapses, for example, the
cells of the brain signal their neighbors by emptying bladders of
chemical molecules. The molecules then jostle around until they
bind to sensor molecules on the neighboring cell, sometimes
triggering a neural impulse. A chemical synapse makes a slow switch,
and neural impulses move slower than sound. With assemblers,
molecular engineers will build entire computers smaller than a
synapse and a millionfold faster.

Mutation and selection
could no more make a synapse into a mechanical nanocomputer than a
breeder could make a horse into a car. Nonetheless, engineers
have built cars, and will also learn to build computers faster
than brains, and replicators more capable than existing cells.

Some of these replicators will not resemble cells at all, but will instead resemble factories
shrunk to cellular size. They will contain nanomachines mounted
on a molecular framework and conveyor belts to move parts from
machine to machine. Outside, they will have a set of assembler
arms for building replicas of themselves, an atom or a section at
a time.

How fast these replicators can replicate will depend on their
assembly speed and their size. Imagine an advanced assembler that
contains a million atoms: it can have as many as ten thousand
moving parts, each containing an average of one hundred atoms -
enough parts to make up a rather complex machine. In fact, the
assembler itself looks like a box supporting a stubby robot arm a
hundred atoms long. The box and arm contain devices that move the
arm from position to position, and others that change the
molecular tools at its tip.

Behind the box sits a device that reads a tape and provides
mechanical signals that trigger arm motions and tool changes. In
front of the arm sits an unfinished structure. Conveyors bring
molecules to the assembler system. Some supply energy to motors
that drive the tape reader and arm, and others supply groups of
atoms for assembly. Atom by atom (or group by group), the arm
moves pieces into place as directed by the tape; chemical
reactions bond them to the structure on contact.

These assemblers will work fast. A fast enzyme, such as
carbonic anhydrase or ketosteroid isomerase, can process almost a
million molecules per second, even without conveyors and
power-driven mechanisms to slap a new molecule into place as soon
as an old one is released. It might seem too much to expect an
assembler to grab a molecule, move it, and jam it into place in a
mere millionth of a second. But small appendages can move to and
fro very swiftly. A human arm can flap up and down several times
per second, fingers can tap more rapidly, a fly can wave its
wings fast enough to buzz, and a mosquito makes a maddening
whine. Insects can wave their wings at about a thousand times the
frequency of a human arm because an insect's wing is about a
thousand times shorter.

An assembler arm will be about fifty million times shorter than a
human arm, and so (as it turns out) it will be able to move back
and forth about fifty million
times more rapidly. For an assembler arm to move a mere
million times per second would be like a human arm moving about
once per minute: sluggish. So it seems a very reasonable goal.

The speed of replication will depend also on the total size of
the system to be built. Assemblers will not replicate by
themselves; they will need materials and energy, and instructions
on how to use them. Ordinary chemicals can supply materials and
energy, but nanomachinery must be available to process them.
Bumpy polymer molecules
can code information like a punched paper tape, but a reader must
be available to translate the patterns of bumps into patterns of
arm motion. Together, these parts form the essentials of a
replicator: the tape supplies instructions for assembling a copy of the assembler, of the
reader, of the other nanomachines, and of the tape itself.

A reasonable design for this sort of replicator will likely
include several assembler arms and several more arms to hold and
move workpieces. Each of these arms will add another million
atoms or so. The other parts - tape readers, chemical processors,
and so forth-may also be as complicated as assemblers. Finally, a
flexible replicator system will probably include a simple
computer; following the mechanical approach that I mentioned in Chapter 1, this will add roughly
100 million atoms. Altogether, these parts will total less than
150 million atoms. Assume instead a total of one billion, to
leave a wide margin for error. Ignore the added capability of the
additional assembler arms, leaving a still wider margin. Working
at one million atoms per second, the system will still copy
itself in one thousand seconds, or a bit over fifteen minutes -
about the time a bacterium takes to replicate under good
conditions.

Imagine such a replicator floating in a bottle of chemicals,
making copies of itself. It builds one copy in one thousand
seconds, thirty-six in ten hours. In a week, it stacks up enough
copies to fill the volume of a human cell. In a century, it
stacks up enough to make a respectable speck. If this were all
that replicators could do, we could perhaps ignore them in
safety.

Each copy, though, will build yet more copies. Thus the first
replicator assembles a copy in one thousand seconds, the two
replicators then build two more in the next thousand seconds, the
four build another four, and the eight build another eight. At
the end of ten hours, there are not thirty-six new replicators,
but over 68 billion. In less than a day, they would weigh a ton;
in less than two days, they would outweigh the Earth; in another
four hours, they would exceed the mass of the Sun and all the
planets combined - if the bottle of chemicals hadn't run dry long
before.

Regular doubling means exponential growth.
Replicators multiply exponentially unless restrained, as by lack
of room or resources. Bacteria do it, and at about the same rate
as the replicators just described. People replicate far more
slowly, yet given time enough they, too, could overshoot any
finite resource supply. Concern about population growth will
never lose its importance. Concern about controlling rapid new
replicators will soon become important indeed.

Molecules
& Skyscrapers

Machines able to grasp and position individual atoms will be
able to build almost anything by bonding the right atoms together
in the right patterns, as I described at the end of Chapter 1. To be sure, building
large objects one atom at a time will be slow. A fly, after all,
contains about a million atoms for every second since the
dinosaurs were young. Molecular machines can nonetheless build
objects of substantial size - they build whales, after all.

To make large objects rapidly, a vast number of assemblers must
cooperate, but replicators will produce assemblers by the ton.
Indeed, with correct design, the difference between an assembler
system and a replicator will lie entirely in the assembler's
programming.

If a replicating assembler can copy itself in one thousand
seconds, then it can be programmed to build something else its
own size just as fast. Similarly, a ton of replicators can
swiftly build a ton of something else - and the product will have
all its billions of billions of billions of atoms in the right
place, with only a minute
fraction misplaced.

To see the abilities and limits of one method for assembling
large objects, imagine a flat sheet covered with small assembly
arms-perhaps an army of replicators reprogrammed for construction
work and arrayed in orderly ranks. Conveyors and communication
channels behind them supply reactive molecules, energy, and
assembly instructions. If each arm occupies an area 100 atomic
diameters wide, then behind each assembler will be room for
conveyors and channels totaling about 10,000 atoms in cross
sectional area.

This seems room enough. A space ten or twenty atoms wide can hold
a conveyor (perhaps based on molecular belts and pulleys). A
channel a few atoms wide can hold a molecular rod which, like
those in the mechanical computer mentioned in Chapter 1, will be pushed and
pulled to transmit signals. All the arms will work together to
build a broad, solid structure layer by layer. Each arm will be
responsible for its own area, handling about 10,000 atoms per
layer. A sheet of assemblers handling 1,000,000 atoms per second
per arm will complete about one hundred atomic layers per second.
This may sound fast, but at this rate piling up a paper-sheet
thickness will take about an hour, and making a meter-thick slab
will take over a year.

Faster arms might raise the assembly speed to over a meter per
day, but they would produce more waste heat. If they could build
a meter-thick layer in a day, the heat from one square meter
could cook hundreds of steaks simultaneously, and might fry the
machinery. At some size and speed, cooling problems will become a
limiting factor, but there are other ways of assembling objects
faster without overheating the machinery.

Imagine trying to build a house by gluing together individual
grains of sand. Adding a layer of grains might take grain-gluing
machines so long that raising the walls would take decades. Now
imagine that machines in a factory first glue the grains together
to make bricks. The factory can work on many bricks at once. With
enough grain-gluing machines, bricks would pour out fast; wall
assemblers could then build walls swiftly by stacking the
preassembled bricks. Similarly, molecular assemblers will team up
with larger assemblers to build big things quickly - machines can
be any size from molecular to gigantic. With this approach, most
of the assembly heat will be dissipated far from the work site,
in making the parts.

Skyscraper construction and the architecture of life suggest a
related way to construct large objects. Large plants and animals
have vascular systems, intricate channels that carry materials to
molecular machinery working throughout their tissues. Similarly,
after riggers and riveters finish the frame of a skyscraper, the
building's "vascular system" - its elevators and
corridors, aided by cranes - carry construction materials to
workers throughout the interior. Assembly systems could also
employ this strategy, first putting up a scaffold and then
working throughout its volume, incorporating materials brought
through channels from the outside.

Imagine this approach being used to "grow" a large
rocket engine, working inside a vat in an industrial plant. The
vat - made of shiny steel, with a glass window for the benefit of
visitors - stands taller than a person, since it must hold the
completed engine. Pipes and pumps link it to other equipment and
to water-cooled heat exchangers. This arrangement lets the
operator circulate various fluids through the vat.

To begin the process, the operator swings back the top of the vat
and lowers into it a base plate on which the engine will be
built. The top is then resealed. At the touch of a button, pumps
flood the chamber with a thick, milky fluid which submerges the
plate and then obscures the window. This fluid flows from another
vat in which replicating assemblers have been raised and then
reprogrammed by making them copy and spread a new instruction
tape (a bit like infecting bacteria with a virus). These new assembler
systems, smaller than bacteria, scatter light and make the fluid
look milky. Their sheer abundance makes it viscous.

At the center of the base plate, deep in the swirling,
assembler-laden fluid, sits a "seed." It contains a
nanocomputer with stored engine plans, and its surface sports
patches to which assemblers stick. When an assembler sticks to
it, they plug themselves together and the seed computer transfers
instructions to the assembler computer. This new programming
tells it where it is in relation to the seed, and directs it to
extend its manipulator arms to snag more assemblers. These then
plug in and are similarly programmed. Obeying these instructions
from the seed (which spread through the expanding network of
communicating assemblers) a sort of assembler-crystal grows from
the chaos of the liquid. Since each assembler knows its location
in the plan, it snags more assemblers only where more are needed.
This forms a pattern less regular and more complex than that of
any natural crystal. In the course of a few hours, the assembler
scaffolding grows to match the final shape of the planned rocket
engine.

Then the vat's pumps return to life, replacing the milky fluid of
unattached assemblers with a clear mixture of organic solvents
and dissolved substances - including aluminum compounds,
oxygen-rich compounds, and compounds to serve as assembler fuel.
As the fluid clears, the shape of the rocket engine grows visible
through the window, looking like a full-scale model sculpted in
translucent white plastic. Next, a message spreading from the
seed directs designated assemblers to release their neighbors and
fold their arms. They wash out of the structure in sudden
streamers of white, leaving a spongy lattice of attached
assemblers, now with room enough to work. The engine shape in the
vat grows almost transparent, with a hint of iridescence.

Each remaining assembler, though still linked to its neighbors,
is now surrounded by tiny fluid-filled channels. Special arms on
the assemblers work like flagella, whipping the fluid along to
circulate it through the channels. These motions, like all the
others performed by the assemblers, are powered by molecular
engines fueled by molecules in the fluid. As dissolved sugar
powers yeast, so these dissolved chemicals power assemblers. The
flowing fluid brings fresh fuel and dissolved raw materials for
construction; as it flows out it carries off waste heat. The
communications network spreads instructions to each assembler.

The assemblers are now ready to start construction. They are to
build a rocket engine, consisting mostly of pipes and pumps. This
means building strong, light structures in intricate shapes, some
able to stand intense heat, some full of tubes to carry cooling
fluid. Where great strength is needed, the assemblers set to work
constructing rods of interlocked fibers of carbon, in its diamond
form. From these, they build a lattice tailored to stand up to
the expected pattern of stress. Where resistance to heat and
corrosion is essential (as on many surfaces), they build similar
structures of aluminum oxide, in its sapphire form. In places
where stress will be low, the assemblers save mass by leaving
wider spaces in the lattice. In places where stress will be high,
the assemblers reinforce the structure until the remaining
passages are barely wide enough for the assemblers to move.
Elsewhere the assemblers lay down other materials to make
sensors, computers, motors, solenoids, and whatever else is
needed.

To finish their jobs, they build walls to divide the remaining
channel spaces into almost sealed cells, then withdraw to the
last openings and pump out the fluid inside. Sealing the empty
cells, they withdraw completely and float away in the circulating
fluid. Finally, the vat drains, a spray rinses the engine, the
lid lifts, and the finished engine is hoisted out to dry. Its
creation has required less than a day and almost no human
attention.

What is the engine like? Rather than being a massive piece of
welded and bolted metal, it is a seamless thing, gemlike. Its
empty internal cells, patterned in arrays about a wavelength of
light apart, have a side effect: like the pits on a laser disk
they diffract light, producing a varied iridescence like that of
a fire opal. These empty spaces lighten a structure already made
from some of the lightest, strongest materials known. Compared to
a modern metal engine, this advanced engine has over 90 percent
less mass.

Tap it, and it rings like a bell of surprisingly high pitch for
its size. Mounted in a spacecraft of similar construction, it
flies from a runway to space and back again with ease. It stands
long, hard use because its strong materials have let designers
include large safety margins. Because assemblers have let
designers pattern its structure to yield before breaking
(blunting cracks and halting their spread), the engine is not
only strong but tough.

For all its excellence, this engine is fundamentally quite
conventional. It has merely replaced dense metal with carefully
tailored structures of light, tightly bonded atoms. The final
product contains no nanomachinery.

More advanced designs will exploit nanotechnology more
deeply. They could leave a vascular system in place to supply
assembler and disassembler
systems; these can be programmed to mend worn parts. So long
as users supply such an engine with energy and raw materials, it
will renew its own structure. More advanced engines can also be
literally more flexible. Rocket engines work best if they can
take different shapes under different operating conditions, but
engineers cannot make bulk metal strong, light, and limber. With
nanotechnology, though, a structure stronger than steel and
lighter than wood could change shape like muscle (working, like muscle, on the
sliding fiber principle), An engine could then expand, contract,
and bend at the base to provide the desired thrust in the desired
direction under varying conditions. With properly programmed
assemblers and disassemblers, it could even remodel its
fundamental structure long after leaving the vat.

In short, replicating assemblers will copy themselves by the ton,
then make other products such as computers, rocket engines,
chairs, and so forth. They will make disassemblers able to break
down rock to supply raw material. They will make solar collectors
to supply energy. Though tiny, they will build big. Teams of
nanomachines in nature build whales, and seeds replicate
machinery and organize atoms into vast structures of cellulose,
building redwood trees. There is nothing too startling about
growing a rocket engine in a specially prepared vat. Indeed,
foresters given suitable assembler "seeds" could grow
spaceships from soil, air, and sunlight.

Assemblers will be able to make virtually anything from common
materials without labor, replacing smoking factories with systems
as clean as forests. They will transform technology and the
economy at their roots, opening a new world of possibilities.
They will indeed be engines of abundance.